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An Introduction to Meta-analysis
Will G Hopkins
Victoria University of Melbourne; Norwegian Sport University (NIH);
High Performance Sport NZ; Auckland University of Technology
 What is a Meta-Analysis?
 Definition, weighted average, heterogeneity, mixed-model metaregression
 Limitations to Meta-Analysis
 Individual differences or responses, publication bias
 How to Do a Meta-Analysis
 Generic measures, finding effects, study characteristics, study
quality, weighting factor, model, publication bias
 Summary and References
What is a Meta-Analysis?
 A systematic review of literature to address this question:
on the basis of the research to date, how big is a given effect,
such as…




the effect of endurance training on resting blood pressure;
the effect of bracing on ankle injury;
the effect of creatine supplementation on sprint performance;
the relationship between obesity and habitual physical activity.
 It is similar to a simple cross-sectional study, in which the
subjects are individual studies rather than individual people.
 But the stats are a bit harder.
 A review of literature is a meta-analytic review only if it
includes quantitative estimation of the magnitude of the effect
and its uncertainty (confidence limits).
 The main outcome is the overall magnitude of the effect.
 The outcome in each study and the meta-analyzed mean
outcome is often shown in a forest plot:
Study1
Study2
Study3
Study4
Study5
Study6
Study7
Study8
Study9
Study10
Study11
Study12
Study13
Study14
Study15
Data are means and
95% confidence
intervals
Mean
harmful
-2
-1
trivial
0
beneficial
1
2
Effect on power output (%)
3
 The main outcome is not a simple average of the magnitude in
all the studies.
 Meta-analysis uses the standard error to give more weight to
studies with more precise estimates.
 The standard error is the expected variation in the effect if the
study was repeated again and again.
 The weighting factor is 1/(standard error)2.
 Other things being equal, use of this factor is equivalent to
weighting the effect in each study by the study's sample size.
 So, for example, a meta-analysis of 3 studies of 10, 20 and 30
subjects each amounts to a single study of 60 subjects.
 For controlled trials, this factor also takes into account
differences in standard error of measurement between studies.
 You can and should allow for real differences or heterogeneity in
the magnitude of the effect between studies.
 The I2 statistic quantifies % of variation due to real differences.
 In early (fixed-effects only) meta-analysis, you did so by testing
for heterogeneity using the Q statistic.
 The test has low power, so you used p<0.10 rather than p<0.05.
 If p<0.10, you excluded "outlier" studies and re-tested, until p>0.10.
 When p>0.10, you declared the effect homogeneous.
• That is, you assumed the differences in the effect between studies
were due only to sampling variation.
• Which made it easy to calculate the weighted mean effect and its p
value or confidence limits.
 But the approach was unrealistic, limited, and suffered from the
problem of whether statistical non-significance means negligible.
 In random-effect meta-analysis, you accept there are always real
differences between all studies in the magnitude of the effect.
 The random effect is the standard deviation representing the
variation in the true magnitude from study to study.
 You get an estimate of this SD and its precision.
 The mean effect ± this SD is what folks can expect typically in
another study or if they try to make use of the effect.
 Include extra random effects when some studies provide >1 effect.
 Don't bother with I2 and Q statistics.
 A better term is mixed-model meta-analysis or meta-regression.
 You include study characteristics as fixed effects.
 The study characteristics will partly account for differences in the
magnitude of the effect between studies.
• Example: differences between studies of athletes and non-athletes.
 The random effect now represents residual variation in the effect
between studies (i.e., not explained by the study characteristics).
 The analysis requires custom software or an advanced stats
package (e.g., SAS).
Limitations to Meta-Analysis
 It's focused on mean effects and differences between studies.
 But what really matters is effects on individuals.
 So we should also quantify individual differences or responses.
 These can be expressed as standard deviations, but researchers
usually don't provide enough info to allow their meta-analysis.
 Inclusion of mean subject characteristics (e.g., age, gender,
genotype) as predictors in the meta-analytic model only partly
addresses this problem.
• It would be better if researchers made available all data for all
subjects, to allow individual patient-data meta-analysis.
 A meta-analysis reflects only published effects.
 But statistically significant effects are more likely to get published.
 Hence published effects are biased high.
 Funnel or related plots can be used to reduce publication bias.
How to Do a Meta-Analysis: Opt for a Generic Measure
 You can combine effects from different studies only when
they are expressed in the same units.
 In most meta-analyses, the effects are converted to a generic
dimensionless measure. Main measures:
 standardized difference or change in the mean (Cohen's d);
• Other forms are similar or less useful (Hedges' g, Glass's )
 percent or factor difference or change in the mean;
 correlation coefficient and slope;
 risk, odds, hazard and count ratios.
Standardized Difference or Change in the Mean
 Express the difference or change in the mean as a fraction of
the between-subject standard deviation (mean/SD).
 Also known as Cohen's d (d stands for difference).
 This example of the effect of a treatment on strength shows
why the SD is important:
Trivial effect (0.1x SD) Very large effect (3x SD)
post
pre
strength
post
pre
strength
 The mean/SD are biased high for small sample sizes and
need correcting before including in the meta-analysis.
 A problem with standardization:
 Study samples are often drawn from populations with different
SDs, so some differences in effect size between studies will be
due to the differences in SDs.
 Such differences are irrelevant and tend to mask more
interesting differences.
 The solution:
 Meta-analyze a better generic measure reflecting the biological
effect, usually percent or factor differences or changes.
• Rarely, the raw measure is best; for example, joint angles
representing flexibility.
 Combine the between-subject SDs from the studies selectively
and appropriately, to get one or more population SDs.
 Express the overall effect from the meta-analysis as a
standardized effect using this/these SDs.
 This approach also effectively eliminates the correction for
sample-size bias with standardized effects.
Percent or Factor Difference or Change in the Mean
 The magnitude of many effects can be expressed as a
percent or multiplicative factor that tends to have the same
value for every individual.
 Example: effect of a treatment on performance is +2%, or a
factor of 1.02, regardless of the raw value of the performance.
 For such effects, percent difference or change can be the
most appropriate generic measure in a meta-analysis.
 If all the studies have small percent effects (<10%), use
percent effects directly in the meta-analysis.
 Otherwise express the effects (and their standard errors) as
factors and log-transform them before meta-analysis.
 Back-transform the outcomes into percents or factors.
 Or calculate standardized differences or changes in the mean
using the log transformed effects and logs of factor SD.
 Measures of athletic performance need special care.
 The best generic measure is percent change.
 But a given percent change in an athlete's ability to output
power can result in different percent changes in performance in
different exercise modalities.
 Example: a 1% change in endurance power output produces the
following changes…
• 1% in running time-trial speed or time;
• ~0.4% in road-cycling time-trial time;
• 0.3% in rowing-ergometer time-trial time;
• ~15% in time to exhaustion in a constant-power test.
 So convert all published effects to changes in power output.
 A difficult and time-consuming task; you have been warned!
 See recent meta-analyses by my students and colleagues.
 For team-sport fitness tests, convert percent changes back into
standardized mean changes after meta-analysis.
Correlation Coefficient and Slope
 These measures of association between two numeric variables
are seldom meta-analyzed.
Performance
 Studies with small between-subject SD
r = 0.80
have small correlations, so correlation
suffers from a similar SD problem as
standardized effects.
r = 0.20
 Solution: meta-analyze the slope.
Maximum O2 uptake
 The slope is biased low (degraded)
only by random error in the predictor.
 Adjust for this bias by dividing the slope by the short-term reliability
intraclass correlation coefficient.
 Express the meta-analyzed slope as either…
• a correlation using SD for an appropriate population, or
• the effect of two SD of the predictor in that population.
Risk, Odds, Hazard and Count Ratios
 When the dependent variable is a proportion or count of
something, effects should be expressed as ratios.
 Risk ratio, relative risk, proportion ratio…
 Example: if proportions of inactive and active adults who get heart
disease after 20 years are 25% and 10%, risk ratio = 25/10 = 2.5.
 Odds ratio for these data is (25/75)/(10/90) = 3.0.
 Hazard ratio is the risk ratio for new occurrences in the next brief
instant of time (the "right-now" risk ratio).
 If proportions change with time, their ratio also changes, but the
hazard ratio usually doesn't.
 So, to meta-analyze studies with different time periods, convert any
proportion and odds ratios to hazard ratios.
 Odds ratios from time-dependent case-control studies are already
hazard ratios, if controls were sampled as the cases came in
(incidence density sampling).
 If proportions are time-independent classifications, convert all
effects to odds ratios for meta-analysis.
 Convert meta-analyzed odds ratios back into proportions and
proportion ratios by choosing a sensible proportion for the
reference group.
 If proportions in the two groups in all studies are low (<10%), all
proportion, odds and hazard ratios are effectively equal and
need not be interconverted.
 Count ratios need no special treatment before meta-analysis
(other than log-transformation).
 Express standard errors of ratio effects as ×/÷ factor errors, then
log transform the ratios and errors for meta-analysis.
How to Do a Meta-Analysis: Find and Record Effects
 Do a search of the literature for studies of a specific effect.
 If the effect has been meta-analyzed already…
• You can do another, if the analysis was done badly or if there have
been many new studies since the previous meta.
• Otherwise find another effect to meta-analyze.
 As you assemble the published papers, broaden or narrow the
focus of your review to make it manageable and relevant via…
• design (e.g., only randomized controlled trials), population (e.g.,
only competitive athletes), treatment (e.g., only acute effects)…
 Document your searches, inclusions and exclusions.
 Record each effect magnitude and inferential information
(sample size, p value, confidence limits, SD of change scores).
 Convert effects into values on a single scale of magnitude.
 In studies with a control or other reference group, record the effect
and inferential information in each group to enhance the analysis.
How to Do a Meta-Analysis: Get Study Characteristics
 Record study characteristics that might account for differences
in the effect magnitude between studies.
 Include the study characteristics as covariates in the metaanalysis. Examples:




duration or dose of treatment;
method of measurement of dependent variable;
quality score;
gender and mean characteristics of subjects (age, status…).
• Record separate outcomes for females and males from the same
study.
• Analyze gender as a proportion of one gender; for example, in a
study of 3 males and 7 females, “Maleness” = 0.3).
• Use this approach for all problematic dichotomous characteristics
(sedentary vs active, non-athletes vs athletes, etc.).
How to Do a Meta-Analysis: Assess Study Quality?
 Some meta-analysts score the quality of a study.
 Examples (scored yes=1, no=0):
• Published in a peer-reviewed journal?
• Experienced researchers?
• Research funded by impartial agency?
• Study performed by impartial researchers?
• Subjects selected randomly from a population?
• Subjects assigned randomly to treatments?
• High proportion of subjects entered and/or finished the study?
• Subjects blind to treatment?
• Data gatherers blind to treatment?
• Analysis performed blind?
 Use the score to exclude some studies, and/or…
 Include as a covariate in the meta-analysis, but…
 Some statisticians advise caution when using quality.
How to Do a Meta-Analysis: Get the Weighting Factor
 Calculate the standard error for each effect via one or more of…
 the confidence interval or limits
 the test statistic (t, 2, F)
• F ratios with numerator degrees of freedom >1 can’t be used.
 the p value
• If the exact p value is not given and you can't calculate the
standard error from the data, try contacting the authors for it.
• Otherwise, if "p<0.05", analyze as p=0.05.
• If "p>0.05" with no other info, deal with the study qualitatively.
 SD of change scores (for controlled trials)
• For studies lacking sufficient information to calculate standard
errors, calculate the typical error (standard error of measurement)
in every other study and impute typical errors (and standard errors
via SD of change scores) from these. The spreadsheet for samplesize estimation at Sportscience calculates the typical errors.
How to Do a Meta-Analysis: Develop the Model
 Do a mixed-model meta-regression.
3
Effect on power output (%)
1
beneficial
0
trivial
2
-1
harmful
 Estimate and interpret
the effect for interesting
types of study or mean
subject.
 Show a moderator plot
rather than a forest plot:
 In a mediator plot, the
subject characteristic
is a difference score
or change score.
Data are means and -2
90% confidence
intervals
0
5
10
15
Baseline training (h.wk-1)
20
 For any linear covariate, estimate and interpret the effect of
double the average of between-subject SD from appropriate
studies.
 Double the SD representing the between-study random effect to
interpret its magnitude as the unexplained typical differences in
the magnitude of the effect between settings.
 For effects where there are control or reference groups…
 include each group effect separately, if possible;
 include a within-study random effect to account for the resulting
repeated measurement;
 include fixed effects to estimate uncontrolled effects and effects
relative to control, best-practice or other reference groups.
 Inspect between-subject SD between and within studies for
evidence of individual differences or responses.
How to Do a Meta-Analysis: Deal with Publication Bias
 Some meta-analysts present the effect magnitude of all the
studies as a funnel plot, to address the issue of publication bias.
 Published effects tend to be larger than true effects, because...
• effects that are larger simply because of
non-sig. SE published
missing delete studies
sampling variation have smaller p values,
studies
funnel
• and p<0.05 is more likely to be published.
funnel
of all
of
all
 A plot of standard error vs effect magnitude studies if
studies if
effect >0
effect=0
should have a triangular or funnel shape.
 If some non-significant studies weren’t
value with
effect 0
magnitude huge sample
published, the plot will be asymmetrical.
• The missing studies are generally smaller (therefore larger SE).
 Effect heterogeneity also disrupts the funnel shape.
 So plot standardized residuals (random-effect solution) vs standard
error (not shown) to spot publication bias and also outlier studies.
 Delete studies with larger SE to give a symmetrical plot.
Summary
 Meta-analysis is a statistical literature review of magnitude of
an effect.
 Meta-analysis uses the magnitude of the effect and its precision
from each study to produce a weighted mean.
 Traditional meta-analysis is based unrealistically on using a test
for heterogeneity to exclude outlier studies.
 Random-effect (mixed-model) meta-analysis estimates
heterogeneity and allows estimation of the effect of study and
subject characteristics on the effect.
 For the analysis, the effects have to be converted into the same
units, usually percent or other dimensionless generic measure.
 It's possible to account for publication bias and identify outlier
studies using a funnel plot or residuals plot.
References
 A good source of meta-analytic wisdom is the Cochrane
Collaboration, an international non-profit academic group
specializing in meta-analyses of healthcare interventions.
 Website: http://www.cochrane.org
 Publication: The Cochrane Reviewers’ Handbook (2004).
http://www.cochrane.org/resources/handbook/index.htm.
 But the (free) Cochrane meta-analysis software is too limited.
 Simpler reference: Bergman NG, Parker RA (2002). Meta-analysis:
neither quick nor easy. BMC Medical Research Methodology 2,
http://www.biomedcentral.com/1471-2288/2/10.
 Glossary: Delgado-Rodríguez M (2001). Glossary on meta-analysis.
Journal of Epidemiology and Community Health 55, 534-536.
 Reference for problems with publication bias: Terrin N, Schmid CH,
Lau J, Olkin I (2003). Adjusting for publication bias in the presence
of heterogeneity. Statistics in Medicine 22, 2113-2126.
This presentation is available from:
See Sportscience 8, 2004